This invention relates in general to anesthetic appliances and in particular to a new and useful device for accurately controlling the rate at which oxygen and anesthetic are fed to a patient.
A gas ratio controlling device for anesthetic appliances is disclosed, wherein the throughput ratio of oxygen to anesthetic gas is controlled by means of a control circuit via a control valve depending on the concentration parameters required, while the oxygen ratio in the supply gas must not fall below a preset value.
Anesthetic appliances are equipped with so-called "oxygen ratio controllers" to simplify usage and to reduce the chance of operator's errors. These controllers forcibly reduce the anaesthetic gas throughput when the oxygen concentration reaches a preset bottom value in a way that does not allow the oxygen concentration to fall below this preset oxygen concentration in the gas mixture.
According to German OS No. 33 10 858 such a gas ratio controlling device could be achieved e.g. by mechanically coupling the operating elements for the anaesthetic gas and oxygen adjustment valves so that the ratio of oxygen to anaesthetic gas is fixed independent of the chosen setting. The prior art gas ratio controlling device features also a by-pass line in the oxygen supply line, which can bridge the oxygen adjustment valve, so that the oxygen ratio in the gas mixture supplied to the patient can be increased independently of the coupling of the oxygen adjustment valve with the anaesthetic gas adjustment valve.
Another version of a gas ratio controlling device for anaesthetic appliances is described in the European PS No. 0 039 932. Herein the anesthetic gas is led into an anesthetic gas supply line comprising a pressure reducer and a proportional control valve, which is positioned upstream of the anesthetic gas adjustment valve. The dosed anesthetic gas flows to a common gas mixture outlet via a precision resistor and a flow meter tube for dosage indication purposes. The oxygen is led to this gas mixture outlet via an oxygen supply line, which also comprises a pressure reducer, an adjustment valve, a precision resistor and a flow meter. At the precision resistors the gas flows create dynamic pressures which are fed into a final control element which controls the proportional control valve according to the difference in the two dynamic pressure values so that the supply of anesthetic gas does not exceed a certain value with regard to its ratio to the oxygen supply. Thus it is guaranteed that the patients gas mixture supply contains at least a certain preset oxygen concentration, e.g. 25% by Vol.
Typical doses for such gas ratio controlling devices range from 3 liter/min to 30 liter/min gas mixture supply. Difficulties can arise when a small throughput is required as the correspondingly small dynamic pressures do not allow for a safe or accurate control of the proportional control valve.
Therefore appropriate means have to guarantee that below a smallest adjustment value for the oxygen throughput of, for example, one liter per minute the anesthetic gas supply is completely blocked by the proportional control valve, i.e. the oxygen concentration in the gas mixture rises to 100 Vol %.
Therefore, in forms of narcosis with very small gas mixture flow rate adjustment problems occur when, in certain phases of the anaesthesia, small doses of the gas are required, which should take into account the needs of the patient, losses through leakage plus a certain safety margin. In practice the total gas mixture amount is reduced to values down to 0.5 liter/min. These kinds of narcosis with very small gas mixture flow rates do not allow for the use of any of the prior art oxygen ratio controllers, as they can not be controlled with adequate safety.
While at a gas mixture flow rate of one liter per minute the flow rate of the anesthetics gas system alone of about 5 minutes is relatively long and there is therefore enough time for the recognition and the correction of maladjustments, the danger potential increases with higher gas mixture flow rates and can be tolerated no longer.
It is necessary that the gas ratio control devices described above are constructed in a way that guarantees a constant safe operation with adjustable, small amounts of anesthetic gas and with higher gas mixture flow rates.